Office workstations are usually equipped with a workstation computer, also called a PC, and a telephone, with said workstation computer generally being connected to a packet-switched, local data network referred to as a LAN (LAN: Local Area Network). The workstation computer furthermore frequently supports what is termed the TCP/IP protocol (TCP: Transmission Control Protocol, IP: Internet Protocol) for data communication with other workstation computers or service-providing server computers, with said computers being addressed within the data network via IP addresses.
The telephone, if forming part of a packet-switched communication system, is customarily also addressed via IP addresses and is referred to frequently as an IP telephone or IP terminal.
The use of IP telephones is advantageous to the extent that an existing infrastructure of a data network already in place can be employed.
When an IP telephone and workstation computer are employed at an office workstation, it is customary for two IP addresses (for the workstation computer and for the IP telephone) to be set up in the network for each office workstation, whereas only a single IP address for the workstation computer is required when circuit-oriented telephones of a circuit-oriented communication system are used.
Only a limited number of IP addresses can be assigned in data networks and their subnetworks. It can consequently be disadvantageous in data networks having an especially large number of IP-based terminals if too many IP addresses have to be allocated and hence few or no free IP addresses are available.
Owing to such limitations in the number of IP addresses in a data network, subnetworks are frequently formed whose IP addresses are not made public; they are only valid locally and so cannot be addressed from outside the subnetwork. Private IP addresses of said type can be multiply assigned because the same IP address can be used in different subnetworks. What is disadvantageous therein is that computers or IP telephones having such private IP addresses cannot be addressed from outside the subnetwork.
It is further known that, for example, computers or terminals in the data network are frequently each assigned an IP address dynamically immediately on being switched on and/or activated. Dynamically assigned IP addresses of said type are usually released again when the relevant computer or device is deactivated and/or switched off. The assigning and/or releasing of IP addresses is frequently performed by what is termed a DHCP server (DHCP: Dynamic Host Configuration Protocol).